(Updated 3:56 p.m.) Gov. Dannel P. Malloy vetoed a campaign finance disclosure bill Friday, saying it likely violated the U.S. Constitution and included provisions representing poor public policy choices.

The bill was an attempt to respond to the U.S. Supreme Court decision in the Citizens United case that allowed corporations, unions, and special interest groups to funnel unlimited funds into political campaigns. It aimed to shed light upon the election process by requiring corporations to disclose their campaign activity.

In his veto message, Malloy said he didn’t want his rejection of the bill to be interpreted as an endorsement of the Citizens United decision, which he opposes.

“While I have advocated for transparency in the elections and campaign finance process for a long time, and could certainly support sensible reform in this area again, I cannot support the bill before me given its many legal and practical problems,” he wrote.

Malloy said those problems include a section that defines independent expenditures so broadly that it applies to almost any communication attempting to influence a public official running for office within 90 days of an election.

“In other words, issue advocacy — not just communication expressly advocating for the nomination, election, or defeat of a clearly identified candidate, or the passage or defeat of a ballot issues — is considered an ‘independent expenditure’ subject to the disclosure provisions of this bill,” he said.

Malloy said that would require nonprofit groups and media outlets to identify their top five donors if they engage in issue advocacy within 90 days of the election, which could dissuade people from supporting the groups.

The bill also may be unconstitutional in light of a court decision which upheld the NAACP’s right to keep its individual donors anonymous, he said.

The American Civil Liberties Union of Connecticut opposed the bill for similar reasons. On Friday Andrew Schneider, ACLU Connecticut’s executive director, issued a press release commending Malloy’s decision, which he said gives lawmakers a chance to craft a more balanced bill.

“This is a very principled stand for the governor to take, and not necessarily an easy one, “ Schneider said. “We appreciate his leadership on this issue.”

The veto was also applauded by Senate Republican leader John McKinney, who agree with Malloy that the bill was unconstitutional.

The Malloy administration signaled early on that it had significant concerns about how the legislation would stand up in court. During the last week of the legislative session the bill was expected to be raised on several different days, only to be tabled as lawmakers and the administration tried to negotiate an agreement.

In the end, legislative leaders moved forward and passed the bill even as Malloy’s top lawyer Andrew McDonald hinted at a veto.

“The governor’s office sought changes to the bill but those issues with which we were concerned have not been modified,” McDonald said as House members debated the bill.

The legislation actually began life as a Malloy proposal but it was reworked in the Government Administration and Elections Committee. The governor’s original proposal would have modified the state’s program for publicly funded campaigns.

Currently, candidates receiving public funds are limited to raising just enough money to qualify for a state grant, but Malloy’s proposal would have allowed them to accept unlimited amounts from private sources if their opponents spend more than the state gave them.

Malloy, who used public funding during his gubernatorial campaign, said he believed allowing publicly funded candidates to raise more money is the only way to keep the public financing system intact in the post-Citizens United world.

“I’m trying to preserve a public system,” he said in March. “I’m willing to live by those rules but you can’t live by those rules and have other people come and destroy the value of those rules. There’s got to be a consequence.”

But in March, the Government Administration and Elections Committee heard from an expert panel on corporate influence in elections, which cautioned against raising the contribution limit.

“Raising contribution limits in response to outside spending is red flag under the Supreme Court rulings and I think very likely unconstitutional in addition to being bad policy,” election law attorney Donald J. Simon told the committee.

The committee subsequently removed the provision from the legislation.

Malloy’s rejection of the bill on Friday didn’t come as a surprise for legislative leaders, who said earlier in the week they were anticipating a veto. House Majority Leader Brendan Sharkey said lawmakers reached out to the governor’s office hoping to negotiate some sort of agreement to be voted upon during Tuesday’s special session, but they were told the administration’s concerns were too numerous to be addressed.

Cheri Quickmire, executive director of Common Cause Connecticut, issued a press release saying the group was “extremely disappointed” by the veto.

“Common Cause believes [Malloy] has squandered an opportunity to pass the strongest disclosure bill in the country,” Quickmire said. “Unfortunately, the citizens of Connecticut will now be left in the dark about who is funding the millions of dollars that will be spent in Connecticut state races this fall, which leaves our state open to the corrupting influence of independent expenditures.”

Karen Hobert Flynn, vice president of Common Cause, questioned Malloy’s desire to see a strong disclosure law on the books, saying his track record on the issue hasn’t been strong.

“The governor’s addition to the GAE disclosure bill would have undermined reforms currently on the books by wiping out all contribution limits if a candidate was the victim of independent expenditures,” Hobert Flynn said. “That would have meant that a corporation or other special interest would have been able to give a multi-million dollar contributions to his campaign.”

Roy Occhiogrosso, Malloy’s senior communications adviser, said the Common Cause statement was “ridiculous” considering Malloy was the first governor elected under the public financing system.

“He committed to it despite the fact that neither of his opponents would, committed to it despite the fact that he was being outspent by huge amounts of money. He got elected governor as a result. So we don’t really need a lecture on campaign finance,” he said.

Though the legislature is expected to meet for a veto override session later in the month, Sharkey said he doesn’t have the two-thirds majority it would take to revive the bill. Asked if the issue can be addressed next year, Sharkey expressed concern that many lawmakers won’t survive the election in the absence of the bill.

“By that time, whoever’s left after November . . . I mean next year is too late,” he said.

Government Administration and Elections Committee Co-Chairwoman Sen. Gayle Slossberg said she also thought the bill’s veto will have an impact on the November elections. Slossberg said she planned to continue to advocate for the passage of a stronger disclosure law before November.